1. Field of the Invention
The present invention relates to an optical transmission line including a connected part of optical fibers and a method of connecting optical fibers at least one of which has a hollow region.
2. Description of the Background Art
Japanese patent 3072842 discloses a single-mode optical fiber having a core which is hollow (hollow core) and a cladding which is provided around the core and which has a photonic band gap structure, such that light propagates by means of the Bragg reflection from the diffraction grating.
The optical fiber is occasionally connected with a usual optical fiber having a core whose refractive index is greater than the refractive index of the cladding, for the purpose of optical communication or light signal processing. Because of the difference in the mechanism for guiding light between a usual optical fiber and an optical fiber having a hollow core, the field intensities of the propagated light are substantially different. Accordingly, the connection loss increases in the case in which their connection is made only by butting the optical fiber having a hollow core and the usual optical fiber. However, in the disclosure of the Japanese patent, no consideration is given at all with respect to connection.
An object of the present invention is to reduce connection loss in a connection of optical fibers having different refractive index profiles, at least one of which has a hollow region.
In order to achieve such object, an optical transmission line includes a connected part which is provided for connecting a first optical fiber having a first core region and a first cladding region with a second optical fiber having a second core region and a second cladding region, wherein either or both of the second core region and the second cladding region has a hollow region which extends along the second optical fiber and the connected part of the hollow region is provided with a refractive index matched substance the refractive index of which is greater than that of the hollow region. Here, xe2x80x9ca refractive index matchedxe2x80x9d means xe2x80x9ca refractive index is adjusted to become approximately equal to the refractive index of the silica glassxe2x80x9d.
In one embodiment, the cross-sectional area of the substance on a plane which is perpendicular to the longitudinal axis of the second optical fiber may decrease continuously as it is more distanced from the connected end face of the second optical fiber. The substance may be liquid. The liquid may be hardened after the connection work. The hardened substance may also be filled between the connected end of the first optical fiber and the connected end of the second optical fiber. The connected part may be provided with a member for reinforcing the connection.
A connecting method is also provided for connecting the first optical fiber and the second optical fiber in an optical transmission line, in which method a refractive index matched substance having a refractive index which is greater than that of the hollow region is put in the connecting end portion of the hollow region of the second optical fiber connected with the first optical fiber.
The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.